Process for coating substrates in strip form

ABSTRACT

A METHOD OF COATING A STRIP OF VISCOUS MATERIAL ON A MOVABLE SUBSTRATE BY FEEDING THE MATERIAL THROUGH A NARROW GAP BETWEEN FEED MEANS AND A MOVABLE SUBSTRATE UNDER A STATIC PRESSURE DIFFERENTIAL BETWEEN THE INLET AND OUTLET OF THE GAP. THE GAP IS SEVERAL TIMES WIDER THAN THE WET FORMED LAYER AFTER FORMATION. IN THE INITIAL LAYER FORMING STEPS AND IN THE CONCLUDING LAYER FORMING STEPS THE HEIGHT OF THE GAP FROM THE SUBSTRATE IS REDUCED SO THAT IT IS SPACED ABOUT TWICE THE THCKNESS FROM THE SUBSTRATE. IN THE LAYER FORMING OPERATION BETWEEN THESE   INITIAL AND CLOSING STEPS THE GAP IS RAISED IN HEIGHT IN ACCORDANCE WITH THE NORMAL STATIC PRESSURE DIFFERENTIAL. A UNIFORM THICKNESS OF THE FORMED LAYER IS OBTAINED.

Aug. 8, 1972 P. HERZHOFF ET 3,682,679

PROCESS FOR COATING SUBSTRATES IN STRIP FORM Filed Feb. 10, 1969 United States Patent "ice US. Cl. 11734 2 Claims ABSTRACT OF THE DISCLOSURE A method of coating a strip of viscous material on a movable substrate by feeding the material through a narrow gap between feed means and a movable substrate under a static pressure differential between the inlet and outlet of the gap. The gap is several times wider than the wet formed layer after formation. In the initial layer forming steps and in the concluding layer forming steps the height of the gap from the substrate is reduced so that it is spaced about twice the thickness from the substrate. In the layer forming operation between these initial and closing steps the gap is raised in height in accordance with the normal static pressure differential. A uniform thickness of the formed layer is obtained.

Reference is made to the copending US. applications Ser. Nos. 797,808 filed Feb. 10, 1969; 797,809 filed Feb. 10, 1969 and 797,800 filed Feb. 10, 1969, now Pat. No. 3,638,604.

The applicants in US. applications Ser. Nos. 797,808 and 797,800 are the same as the present application and the applicants in US. application Ser. No. 797,809 are the same as the present application together with Rolf Behr.

This invention relates to a process for coating substrates in strip form with viscous solutions, preferably with photographic emulsions, in which the substrate is wetted in a sealed coating chamber whose end at that point where the emulsion emerges is formed by a narrow gap several times wider than the wet, finished layer is thick, and in which, to counteract the forward movement of the moved substrate, layer formation takes place under an adjustable static pressure differential between the inlet end and the outlet end of the gap, the lower pressure prevailing at the inlet end.

The object of the invention is to prevent the required thickness of the wet layer from being exceeded both at the beginning and the end of coating.

In the figure, a strip 1 is guided over a coating roller 2 and passes a rear wall 3 of a coating machine 4. In a coating chamber 5 of the coating machine 4 the strip 1 is wetted with a coating solution. A front coating wall 6 has a layer forming edge which forms a layer forming gap 7 which in conjunction with the roller 2 and strip 1 determines the thickness of a layer 8 on the strip 1. The coating chamber 5 communicates through a pipe 9 with a supply vessel 10 whereby a pressure differential within said gap is created. The front coating wall 6 is slidably mounted in the coating machine 4 and is reciprocable by the eccentric 11.

When the strip is being coated, layer formation takes place under the efiect of a static pressure differential between the inlet end and the outlet end of the layer-forming gap. At the beginning of coating, i.e. when the strip 3,682,679 Patented Aug. 8, 1972 is wetted for the first time in the sealed coating chamber, and at the moment the coating is applied, the static differential which is automatically formed through controlled introduction of the emulsion, passes through all the values between zero and the value associated with the required thickness of the wet layer. Corresponding to the fluctuating static pressure differential in the transfer zone both at the beginning and the end of coating, the layer thickness of the wet layer passes through the values from half the height of the gap, where there is no pressure differential to the required thickness of the layer thereby forming wedges of coating.

This factor is a considerable disadvantage so far as the application of the process is concerned. The fairly thick layers at the beginning and at the end of coating, call for a greater degree of drying efliciency on the part of the coating machine than the actual process of coating itself. This means that, with the coating machine operating at a given level of drying efiiciency, the potential rate of production, based on the required layer thickness of coating, has to be reduced in view of the local increases in layer thickness. It is not possible to compensate for these increases by increasing the speed for the period of time between when the increase in layer thickness at the beginning of coating has passed the dryer and before the end enters the dryer, because changes in speed of this order cannot be made without impairing the quality of coating, producing intolerably high losses.

vIt has now been found that this undesirable reduction in output can be avoided if, before the beginning of coating and before the coating is applied, the height of the layer-forming gap is reduced to at least twice the required thickness of the wet layer for a period of time at the beginning and at the end of coating.

A triangular flow profile of the emulsion in the layerforming gap is associated with a zero value of the static pressure differential at this gap. A layer thickness of the coating equal to half the height of the gap results from this flow profile. Accordingly, it is possible by reducing the height of the gap to the extent specified to guarantee a layer thickness of the coating which is equal to or smaller than the required layer thickness which accordingly does not result in a need for any increase in the drying capacity either for the beginning or for the end of coating or, as a consequence, in any reduction in output in the coating machine. At the beginning of coating, the transition from the reduced gap height to the normal gap height required for permanent operation should take place steadily and in accordance with the increasing static pressure differential at the layer-forming gap. At the end of coating the gap height should be changed steadily from the normal gap height to the reduced gap height in accordance with the diminishing pressure differential.

One example of a suitable apparatus for modifying the height of the layer-forming gap is an eccentric which determines the gap between the layer-forming edge and the substrate supported by the coating roller.

What is claimed is:

1. In a process for coating a fluid photographic emulsion of uniform and controllable thickness on a moving substrate comprising the steps of wetting the substrate in a gap by picking the coating emulsion from a body of the photographic emulsion contained in a chamber subject to a hydrostatic pressure difierence created by the moving substrate picking up the coating emulsion from the body of the emulsion at the lower pressure prevailing Where the substrate is first wetted at the gap between the substrate and an adjacent surface of the chamber which is in the system, the width of the gap being several times wider than the thickness of the coating on the substrate, the gap having an entrance side and an exit side with the emulsion body in the chamber at the entrance side feeding the emulsion from a supply in a completely sealed self-communicating pipe system to the gap, removing the emulsion from the gap onto the moving substrate, lowering the level of the emulsion in said supply by said removal of the emulsion and creating said pressure differential within said gap between the side closest to the chamber and the side where the substrate leaves the gap the emulsion being metered to the supply so that at an established rate of travel of forward movement of the substrate a static pressure diiferential is attained in the gap governing the thickness of the resultant coating, the improvement which comprises reducing the gap Width to at least twice the value of the desired thickness of the coating at the established rate of travel, before the web is traveling, steadily increasing the gap width during the initial stage of coating concomitantly with the steadily increasing static pressure differential in the gap, until the pressure differential at the established rate of travel is reached, which governs the thickness of the coating, so that a layer of constant thickness is formed during the initial stage of coating and the established rate of travel of forward movement of the substrate.

2. The process according to claim 1 and adding the step of steadily increasing the gap width during a final stage of coating according to the steadily decreasing pressure differential in the gap to at least twice the value of the wet coating at the established rate of travel of forward movement of the substrate.

References Cited UNITED STATES PATENTS WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R. 117-111, 119 

